1. Technical Field of the Invention
The present invention relates to an ND filter and an aperture diaphragm apparatus. An ND (Neutral Density) filter is used in a light quantity diaphragm for the purpose of uniformly attenuating the quantity of transmitted light in an overall visible wavelength range. The present invention relates particularly to a density variable ND filter in which a transmission density is continuously changed.
2. Prior Art
In a conventional photographing system, when the luminance of an object is too high, an excessive quantity of light greater than a predetermined quantity might enter into a photosensitive face through the diaphragm even if the diaphragm is adjusted to the smallest diameter (namely, the aperture diameter is adjusted to the smallest). Therefore, practically an ND filter is mounted on the image pickup system to control the quantity of light that enters into the photosensitive face. In this case, the spectral characteristics of the ND filter is designed flat to simply reduce the quantity of incident light hence the uniform transmittance is provided for the overall visible wavelength range as the basic optical performance of the ND filter.
An ND filter having a uniform transmission density has been employed as the light quantity diaphragm of a camera. Recently, an ND filter in which the transmission density is stepwise varied in divided zones is employed as an advanced filter. Furthermore, for a video optical system that requires sequential photographing, a density variable ND filter in which the transmission density is continuously changed is demanded, because a high resolution can be obtained in a wide range of brightness by use of the density variable ND filter, as compared with a single-density ND filter or a multi-density ND filter. The density variable ND filter is disclosed in the following patent documents.
[Patent Document 1] JP-A-Hei 6-95208
[Patent Document 2] JP-A-Hei 10-96971
[Patent Document 3] JP-B-Hei 2-47722
[Patent Document 4] JP-A-2003-043211
According to patent document 1, a master pattern of a ND filter is prepared, which has a reflection density distribution having a predetermined relationship with a transmission density distribution of an ND filter that provides a filtering function for transmitted light when the ND filter is mounted on the aperture blade of an aperture diaphragm apparatus. This master pattern is photographed by a camera using a film which has the transmittance through a film base equal to or greater than 80% and which has a halation prevention layer. Then, the film is developed and is employed as an ND filter. According to patent document 2, by a method of manufacturing an ND filter that is provided for the aperture blade of an aperture diaphragm apparatus, a plastic film containing organic pigments is irradiated by light of high energy, while the amount of irradiated light is changed partially, so that an ND filter having a variable density distribution is prepared. According to patent document 3, an ND filter employs a metal material as an absorption film, and the quantity of transmitted light is continuously changed radially from the center where reflection prevention films are formed. In this ND filter, the absorption film is divided into multiple thin layers, and the divided absorption layers are inserted into the boundaries of the reflection prevention films.
According to patent document 1, the change of the density of the ND filter is provided by differentiating the precipitated amount of silver particles. However, light scattering due to silver particles causes the deterioration of the resolution, and the use of this type of the ND filter is difficult for a recent image pickup system for which a resolution has been increased. For patent document 2, the color decay of the organic pigments, especially, the change of the transmission characteristics under high temperature and high humidity environment is a serious drawback. According to patent document 3, the change of the density is provided by locally inserting metal films having different thicknesses between the reflection prevention films. However, according to this method, the uniform transmission characteristics in the overall visible light wavelength range cannot be obtained, and therefore it is difficult to apply this type of ND filter to the image pickup system.